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      <td class="header">Coregistration</td>
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<h3>Coregistering&nbsp;InSAR&nbsp;Products</h3>

<p> The sub-pixel coregistration of SAR images is a strict requirement
and critical component of any interferometric processing chain. It is
an essential step for the accurate determination of phase difference,
and applications such as DEM map generation, interferometric
deformation analysis, etc.
</p>

<p><br>
The interferometric modules of the toolbox will
accurately&nbsp;co-register one&nbsp;or more&nbsp;slave images with
respect to a master image. The co-registration procedure is completely
automatic. Apart from defining the processing parameters, no additional
input nor intervention from the user is required. For example the
distribution of correlation (optimization) windows are done in
automatic manner for both master and slave image. Also, the refinement
of the coregistration offsets is done in a fully automatic way,
including downloading and interpolation of the a-priori
digitial-elevation-model.
</p>

<h4>Brief Implementation Details</h4>

<p><br>
The implementation of the coregistration procedure is based on the
cross-correlation technique. Since this technique for an optimal
alignment tend to be slow for very large search windows, the procedure
is usually separated in two main steps: <i>coarse</i> and <i>fine</i>
coregistration. In the coarse coregistration, the offsets are
approximated either by using the satellite orbits and timing as a
reference, and/or by defining an approximate common points in
master/slave images and performing correlation matching with large
windows. The subsequent fine coregistration applies automation
correlation technique to obtain sub-pixel alignment accuracy. After the
coregistration offsets are computed, the estimation of the
coregistration polynomial (CPM) and interferometric resampling of slave
images to the master geometry is performed.
</p>

<h4>Overview of Coregistration Processing Chain</h4>

<p><br>
The interferometric coregistration is performed by create stack, coarse
fine coregistration and resampling.
</p>

<br>

<img style="" alt="" src="images/insar_coregistration_diagram.jpg">
<br>

<br>

<div style="margin-left: 20px;"><b>Figure 1.</b> Flowchart of
inteferometric co-registration<br>
</div>

<h4>Input Images and Data Support</h4>

<p><br>
Input SAR images may be fully ("full frame") or only partially
overlapping ("subset"), they have to be from acquisitions taken at
different times using compatible, in the interferometric sense,
sensors, and input images must belong to the same type (i.e., them must
be complex).
</p>

<p><br>
While in principle the implementation of the InSAR coregistration is
flexible enough to allow processing of real (detected) products, for
now only complex (single-look-complex) data is supported.
</p>

<h4>Create Stack</h4>

<p> The Create Stack operator collocates the master and slave
images based into a single reference (master) geometry. Basically the
slave image data is subset into geometry of the master image. With
performing this operation the master and slave images share the same
geo-positioning information, and have the similar dimensions. For
overlap and geometry calculation either orbital data, or annotated
tie-point-grids (i.e., ground-control-points) can be used. In other
words the coarse coregistration is performed using orbital information
or annotated GCPs. The method based on orbits is recommended for all
platforms, since especially in case of old sensors (ERS1/2) annotated
GCPs prove not to be reliable through-out the whole mission lifetime.
</p>

<p><br>
More details on this operator are given in the operator help - <a href="CreateStackOp.html">Create Stack</a>.
</p>

<h4>Cross Correlation</h4>

<p><br>
The Cross Correlation operator creates an alignment
between master and slave images by matching automatically distributed
correlation optimization windows to their corresponding slave windows.
There are two steps: coarse and fine registration. The offsets between
master and slave are computed by maximizing the cross-correlation
between master and slave images on a series of imagettes defined across
the images. First on coarse level, with large windows and lower
oversampling factors, later on fine level, with smaller windows and
higher oversampling factors.
</p>

<p><br>
For details and specifics on the operators input parameters, readers
are referred to operators help <a href="CrossCorrelationOp.html">Cross
Correlation</a>.
</p>

<h4>Warp</h4>

<p><br>
With the master-slave offsets computed, a coregistration polynomial
(CPM) is estimated by the Warp operator, which resamples pixels in
the slave image into pixels in the master image.
</p>

<p><br>
This resampling is performend in two-steps: (1) reconstruction of the
continuous signal from its sampled version by convolution with an
interpolation kernel, and (2) sampling of the constructed signal at the
new sampling locations.
</p>

<p><br>
For details of the Warp operator, readers are referred to <a href="WarpOp.html">Warp operator</a>.
</p>

<h4>Coregistration with SNAP<br>
</h4>

<p><br>
Processing steps that are listed below should give satisfactory results
for most of the interferometric combinations.
</p>

<ol>

  <li>From the <span style="font-style: italic;">Radar</span> menu, in
sub-menu <i>Coregistration</i>,
select <i>Coregistration</i>.</li>
  <li>Add a master product to the ProductSet Reader.</li>
  <li>Add any slave products to the ProductSet reader or drag and drop
products from a Project.</li>
  <li>In the CreateStack tab, select no resampling for InSAR products.
A resampling method may be selected for detected product which are of
differnt passes.<br>
  </li>
  <li>In the Cross Correlation enter the number of
correlation optimization windows to create.</li>
  <li>In the Warp tab select the thresholds and coregistration
polynomial order.</li>
  <li>In the Write tab enter in the output product name and location.</li>
  <li>Press the Run button to begin processing.</li>
</ol>

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